In recent years, movements of reducing the use of a chemical fertilizer and promoting the use of an organic fertilizer have been active worldwide from the viewpoint that a recycling society should be established.
However, in ‘hydroponics’ without the use of soil, which is increasingly used in the production of, for example, vegetables such as a tomato and flower and ornamental plants, the direct addition of an organic material to a nutrient solution generates a harmful intermediary metabolite, which damages the root of a plant. Hence, the utilization of the organic fertilizer in hydroponics has heretofore been unthinkable. Only the chemical fertilizer is therefore used in hydroponics at present.
There is a need for a technology for mineralizing an organic material to a useful inorganic nutrients such as nitrate nitrogen in order that the organic fertilizer is utilized in hydroponics. A conventional technology for mineralizing an organic material is, for example, a waste water treatment technology utilizing microorganisms (see, for example, Patent Literature 1). However, nitrate nitrogen is lost because such technology is accompanied by a denitrification involving reducing produced nitrate nitrogen to release nitrogen gas. Accordingly, the technology does not meet the purpose of manufacturing an inorganic fertilizer.
In view of the foregoing, the multiple parallel mineralization method described in each of Patent Literature 2 and Non Patent Literature 1 has been invented as a technology capable of efficiently collecting nitrate nitrogen (as a nitrate ion) from an organic material to be utilized as inorganic nutrients.
The technology is a highly reproducible method capable of degrading organic nitrogen while suppressing the denitrification, and collecting a nitrate ion which is nitrate nitrogen as inorganic nutrients with high efficiency, and is an unprecedented technology. This has allowed “hydroponics employing an organic fertilizer” and “manufacture of an inorganic fertilizer containing nitrate nitrogen using an organic material as a raw material”, both of which have been hitherto difficult to be achieved (see, for example, Non Patent Literatures 1 and 3).
The invention described in Patent Literature 2 has gained a great deal of attention as a technology for realizing hydroponics employing an organic fertilizer, and manufacturing inorganic nutrients such as nitrate nitrogen using an organic resource as a raw material. Therefore, the invention has had high expectations from, for example, a company which plans to recycle an organic resource in addition to a farm or a plant factory having an interest in the invention as a novel hydroponics technology.
Further, the utilization of the multiple parallel mineralization method described in Patent Literature 2 allowed ‘water culture using an organic fertilizer’ to be performed by directly adding an organic fertilizer to a nutrient solution to produce nitrate nitrogen.
However, when a fertilizer containing inorganic nutrients is manufactured by the multiple parallel mineralization method described in Patent Literature 2, during the degradation of an organic material (ammonification) and the production of nitrate ion (nitrification), nitrification is conducted by only the action of microorganisms attached on a culture tank wall surface in a culture tank, and hence, the surface area of the wall surface determines the rate of a reaction. Accordingly, there has been a problem in that a very long time is required in terms of the rate of a reaction of mineralizing an organic material to nitrate nitrogen.
In addition, the culture and reaction of the microorganisms require constant aeration (operation of constantly keeping an aerobic condition through aeration or the like). Thus, an electrical power cost may be a problem when a large-scale treatment is assumed. Therefore, in the production of nitrate nitrogen as inorganic nutrients from an organic material, there has been a demand for the development of a method capable of remarkably enhancing the rate of a reaction (efficiently) and being performed without requiring aeration and constant electrical power.
By the way, in the case of performing hydroponics using a solid medium such as rockwool which is often utilized in, for example, the cultivation of a tomato ('hydroponics with solid medium cultivation'), there has been a problem in that, when an organic fertilizer is directly added to a nutrient solution, nitrate nitrogen is hardly produced because of the putrefaction of an organic material due to its insufficient degradation (see, for example, Non Patent Literature 2).
In view of the foregoing, studies have been made on the utilization of the multiple parallel mineralization method described in Patent Literature 2. However, also in this case, when an organic fertilizer is directly added to a nutrient solution, an organic component is present in the nutrient solution in a dissolved state, which may cause the clogging in a drip tube or a solid medium, and may cause problems such as the putrefaction of an organic material that has caused the clogging in the solid medium. Hence, there has been a problem in that the method itself is difficult to be put into practical use.
Accordingly, in order that the method described in Patent Literature 2 is utilized to perform solid medium cultivation, it is necessary to completely mineralize an organic material into an inorganic nutrient solution (nutrient solution containing an organic component at as low a concentration as possible) prior to being used for the nutrient solution in solid medium cultivation.
Therefore, in this case, a preparative operation to be performed before cultivation is complicated and requires a long time. Thus, there has been a demand for the development of a technology that allows an organic fertilizer to be employed in solid medium cultivation by a more simple method.
It should be noted that, although methods involving fixing microorganisms on a solid medium have been conventionally known (see, for example, Patent Literature 1), those methods are each accompanied by a denitrification to lose a fertilizer component (nitrate nitrogen), and hence such solid medium has not been able to be utilized to perform hydroponics by directly adding an organic fertilizer.    [Patent Literature 1] JP 2001-300583 A    [Patent Literature 2] JP 2007-119260 A    [Non Patent Literature 1] “Hydroponics using organic fertilizer”, Agriculture and horticulture, Vol. 81, p. 753-764 (2006)    [Non Patent Literature 2] “Management of nutrients and solid media in drip Hydroponics” Hakuyusha, p. 119-155 (2005)    [Non Patent Literature 3] “Export of inorganic fertilizer produced from kitchen garbage and excrements” Research journal of food and agriculture, Vol. 31, p. 44-46 (2008)